The present invention relates to waveguide structures and a method of fabrication thereof and in particular waveguide structures for use with terahertz signals.
Conventional waveguide structures, which have been fabricated for signals up to around 600 GHz, comprise a discrete planar diode mounted on a microstrip of active components which in turn is mounted on a separately fabricated support. Difficulties have been encountered though in scaling down such structures for higher frequency signals due to limitations encountered in mounting the diode on the microstrip and the parasitic capacitive effects resulting from the diode chip/microstrip combination.
The present invention seeks to provide waveguide structures which addresses the limitations and difficulties encountered with those currently available and to provide waveguide structures suitable for use with frequencies in the range 50 GHz-10 THz and preferably 800 GHz-10 THz.
The present invention provides a method of fabricating a hollow metallic structure comprising coating an upper surface of a substrate in an etchable polymeric material, etching the polymeric material to produce a former, coating the surface of the former with a metallic material, and thereafter dissolving the polymeric material to produce a hollow metallic structure.
In a preferred embodiment, the method is used to fabricate a waveguide structure, wherein one or more active components are fabricated on the substrate before the substrate and the active components are coated in the polymeric material, whereby the resultant hollow metallic structure has the one or more active components positioned distant from the base of the hollow structure on a common fabrication plane.
Waveguide channels are formed by joining two hollow structures together to form the channel. Thus, with the present invention waveguide channels may be formed integrally with one or more active waveguide components and at the dimensions needed for use at frequencies in the range 50 GHz-10 THz. The method also enables a waveguide channel to be formed immediately adjacent active components so that the active components are suspended over the channel in the waveguide structure. Also, as semiconductor wafer fabrication techniques are used this enables the waveguide structures to be mass produced.
Differences in height of the required waveguide circuitry is compensated for as the polymer not the substrate is processed which means that there is a common fabrication plane for the active devices. Moreover, this allows complete integration of components operating at different frequencies on the same wafer.
As the waveguide channel is split in the described plane, removal of the polymeric material is simplified and the active components can be suspended in air.
Preferably, an etch stop layer is formed in the wafer so that when the rear of the wafer is etched the etching is prevented from extending into the active components previously fabricated. Also, additional components may be formed on the rear of the wafer after etching of the bulk wafer material.
In addition, the polymeric former may be coated in a thin layer of a metal such as gold before electroforming of the metallic structure is performed. The polymeric material is preferably patternable such as photoresist or PMMA.
In a further aspect the present invention provides a waveguide structure comprising one or more waveguide channels having one or more active components on a semiconductor wafer formed integrally therewith. Ideally, both the one or more waveguide channels and the one or more active components lie in a common plane which is the fabrication plane of the semiconductor wafer. The waveguide structures may be used as sub-harmonic mixers, oscillators, multipliers, amplifiers or detectors, amongst others.
Ideally the waveguide structure is adapted for use with frequencies in the range 50 GHz-10 THz. Preferably, the one or more active components are positioned immediately adjacent and are suspended within the one or more waveguide channels. The waveguide structure may comprise at least two waveguide channels both lying in the fabrication plane of the wafer and arranged at 90xc2x0 or 180xc2x0 with respect to one another. Also, one or more of the waveguide channels may extend to at least one edge of the wafer whereby the one or more channels may be brought into communication with channels formed in further wafers.
This configuration is also particularly suited to the formation and use of microstrip circuitry.